Manipulating intermolecular interactions for ultralong organic phosphorescence

Ultralong organic phosphorescence (UOP) materials have received considerable attention in the field of organic optoelectronics due to their long lifetime, high exciton utilization, large Stokes shift, and so on. Great advancements have been achieved through manipulating intermolecular interactions for high‐performance UOP materials in recent years. This review will discuss the influence of various intermolecular interactions, including π‐π interactions, n‐π interactions, halogen bonding, hydrogen bonding, coordinative bonding, and ionic bonding on phosphorescent properties at room temperature, respectively. We summarize the rule of manipulating intermolecular interactions for UOP materials with superior phosphorescent properties. This review will provide a guideline for developing new UOP materials with superior phosphorescent properties for potential applications in organic electronics and bioelectronics.


INTRODUCTION
Ultralong organic phosphorescence (UOP), namely ultralong room temperature phosphorescence (RTP) of organic molecules, is one type of persistent luminescence, [1][2][3] which lasts for a period of time after the removal of excitation sources at room temperature. [3] In recent years, UOP has attracted considerable attention due to broad applications in anti-counterfeiting, [4] lighting and displays, [5] chemical sensors, [6][7] bioimaging, [8][9][10][11][12] and so on. The Jablonski diagram schematically displays all the electronic states involved in luminescence phenomena. [13] After the electronic absorption from S 0 →S n and fast internal conversion S n →S 1 according to Kasha's rule, the singlet exciton is formed, and then it undergoes intersystem crossing (ISC) to form triplet excitons. [14] The formed triplet exciton undergoes several competing processes, such as the radiative decay to the ground state (T 1 →S 0 ) with phosphorescence emission, non-radiative decay to the ground state, the progress of reversible ISC from the excited triplet to singlet states, and so on. Notably, the triplet excitons are easily quenched by thermal disturbance and oxygen. [15] Therefore, there are two approaches to improving phosphorescence properties. One is promoting the ISC from the S 1 →T 1 . [16] The other is suppression of the quenching of triplet excitons by restricting the non-radiative transitions. To date, a series of feasible strategies have been proposed to obtain UOP, such as crystal engineering, [15,17,18] embedment of small organic molecules into a rigid matrix, [19][20][21][22][23][24] H-aggregation, [4,25] polymerization, [26,27] the formation of carbon dots, [28][29][30][31] the construction of metal-organic frameworks (MOFs), [32,33] and so forth. [34] In the past decades, the mass of UOP materials has been developed, including molecular crystals, amorphous polymers, MOFs, carbon dots, and so on. It is worth noting that the UOP performance is highly dependent on intermolecular interactions. For instance, the phosphorescence properties of organic phosphors in crystal or polymer matrices are better than that in dilute solutions. In the crystal, intermolecular interactions play a critical role in forming a rigid molecular environment, which can efficiently restrict the non-radiative transitions of excitons. [15,35,36] Besides, intermolecular interactions, such as π-π interactions, can promote electronic communications between neighboring molecules, which lowers the energy level of the luminescence materials. Therefore, manipulation of intermolecular interactions has a great impact on the UOP performance by tuning the non-radiative transitions of excitons, energy level, and so on. To obtain excellent phosphorescence properties, researchers have adopted many concise chemical modifications to construct various intermolecular interactions, such as strong π-π interactions for long-lived emission, [37] effective halogen bonding for high phosphorescence efficiency (Φ P ), and so on. [16] Despite great successes in the development of UOP materials with excellent phosphorescence performance during past decades, there is no systematic review of the relationship between intermolecular interactions and phosphorescence properties.
In this review, intermolecular interactions involved in UOP materials have been classified into six parts, namely, ππ interactions, n-π interactions, halogen bonding, hydrogen bonding, coordinative bonding, and ionic bonding. By analyzing chemical structures, intermolecular interactions, and phosphorescent properties of the typical UOP materials, we summarized the influence of different intermolecular interactions on phosphorescent properties. Notably, π-π interactions have a great impact on phosphorescent lifetimes and dynamic phosphorescence behavior. It is worth noting that purely organic compounds have low phosphorescence efficiency due to weak spin-orbit coupling (SOC). The n-π interactions or halogen bonding are usually introduced to promote the SOC, thereby facilitating ISC, thus enhancing phosphorescence quantum yield. To simultaneously enhance phosphorescence lifetime and efficiency, hydrogen bonding can rigidify the molecular conformations and decrease non-radiative transitions of triplet excitons, contributing to ultralong lifetime and high Φ P . Coordinative bonding and ionic bonding are stronger than hydrogen bonding, which builds a more rigid environment. Moreover, ionic bonds also have a good ability to facilitate intramolecular charge transfer for the generation of UOP. Finally, we propose an outlook on the development of UOP materials and potential applications.

CLASSIFICATION OF INTERMOLECULAR INTERACTIONS
To date, various materials with excellent UOP performance have been developed, including organic crystals, [38] hostguest doping systems, [39] polymers, [40] carbon dots, [28,41] MOFs, and so on. [2,3] These materials contain different kinds of covalent bonds and noncovalent intermolecular interactions. Non-covalent intermolecular interactions not only govern their self-assembly or co-assembly process but also regulate the molecular packing arrangements, [42] and thus manipulating the UOP properties of organic materials. Therefore, suitable intermolecular interactions play an important role in achieving ultralong phosphorescence lifetimes and high phosphorescence efficiency (Φ P ). Generally, π-π interactions appeared in π-conjugated chromophores, such as carbazole, dibenzofuran, phenothiazine, and so forth. [2] The close packing style of molecules with a large overlap of π-conjugated chromophores and a short distance of π-π interactions contributes to efficiently suppressing nonradiative transitions, thus prolonging the phosphorescence lifetime. [37] In 2015, An et al. proposed that H-aggregation can stabilize triplet excitons for UOP at ambient conditions, which aroused great attention to developing purely organic materials with RTP. Subsequently, Li and coworkers reported the large overlap of π-π interactions in crystals prolonged the phosphorescence lifetimes due to low excited energy levels and more electron communication between dimers. n-π interactions are usually constructed between aromatic carbonyl/sulfonyl and π-conjugated chromophores ( Figure 1). [43] The lone pair electrons (n) and π electrons have strong SOC to facilitate the ISC. Halogen bonding has been introduced in UOP materials mainly via modifying halogen atoms (Cl, Br, and I) [44] or complexing with anions (Cl − , Br − , and I − ) to promote SOC as well as to reduce the non-radiative transition ( Figure 1). In 2016, Chi and coworkers proposed that intermolecular electronic coupling units between carbonyl groups (n unit) and carbazole group (π unit) can improve Φ P . Carboxylic acid group, phenylboronic acid group, and so forth can form strong intra/intermolecular hydrogen bonds (Figure 1), [45][46][47][48] which creates an extremely rigid environment for UOP with good performance. For instance, a three-dimensional hydrogen bonding network between melamine and aromatic acids provides a rigid environment to lock the chromophore, achieving an ultralong lifetime of 1.91 s and a high phosphorescent efficiency of 24.3%. [45] Tian and coworkers achieved UOP in polymer via copolymerization of acrylamide with different phosphors. Polyacrylamide provides multiple hydrogen bonding, which effectively restricts non-radiative transitions. Aromatic carbonyl group, oxygen, nitrogen, and fluorine atoms, as well as aromatic π-electron clouds, provide suitable positions for weak hydrogen bonds (Figure 1), [49,50] which shows a great effect on phosphorescent properties, too. Some organic compounds with carboxylic acid formed complexes with different metals (like Zn 2+ , Cd 2+ , and so forth) via coordinative bonds (Figure 1), [32,33] which can also rigidify the chromophores and be beneficial to the charge transfer. Some organic salts such as carboxylic acid salts, [51] tetraphenylphosphonium cations, [52] phosphonium bromide salts [53] formed ionic bonding with the organic chromophores to generate a highly rigid environment within crystal lattice to reduce molecular motions and vibrations, thus boosting efficient UOP. In 2018, Huang and coworkers reported the first ionic crystals with UOP. [51] Later, Tang and coworkers discovered the lifetimes of crown ethers could be prolonged via the complexation with potassium. [54]

π-π interactions
The organic π-conjugated units have weak SOC between the lowest triplet excited state (T 1 ) and the ground state (S 0 ), [55] resulting in the slow radiative transition from T 1 to S 0 , which is essential to achieve UOP in organic compounds. π-π interaction is the intermolecular interaction between π-conjugated units in crystals, which restricts the nonradiative decay process via suppressing the molecular motions and decreases the quenching of triplet excitons by isolating the external oxygen simultaneously. [15] In other words, π-π interactions in F I G U R E 1 Classification of intermolecular interactions in ultralong organic phosphorescence (UOP) materials

S C H E M E 1
Chemical structures of ultralong room temperature phosphorescence materials based on π-π interactions crystal, featuring the aromatic-aromatic interactions that were classified as edge-to-face, offset face-to-face, and parallel face-to-face stacking, [56] play a crucial role in manipulating UOP properties of π-conjugated organic compounds. In 2015, Huang and coworkers proposed that parallel face-toface H-aggregated stacking in crystal effectively stabilized the triplet excitons for UOP. A series of UOP materials were reported based on triazine derivatives (1-4, 6) with an ultralong phosphorescence lifetime of 1.35 s (Scheme 1). [4] Based on H-aggregation, by introducing the alkoxyl group into triazine-based compounds (5)(6)(7)(8)(9), the phosphorescence lifetimes changed from 1.22 s to 0.93 ms with increasing chain lengths of alkoxyl (Figure 2A), which can be ascribed to manipulating the magnitude of offset in face-to-face stacking via the length of alkoxyl chains, thus tuning phosphorescence lifetime ( Figure 2B). [57] Impressively, compounds 7-9 show dynamic UOP. The phosphorescence lifetime of molecule 8 is prolonged sharply from 1.8 ms to 1330 ms after ultraviolet (UV) light irradiation for 8 minutes under ambient conditions. Similarly, Lucenti et al. synthesized a cyclic triimidazole 10 with H-aggregation molecular packing, together with an isomeric by-product 11. [25] Interestingly, compound 10 emitted UOP with a lifetime of 990 ms, while no phosphorescence was observed from compound 11. X-ray single crystal data revealed that 10 showed strong π-π interaction with short distances of 3.204 Å and 3.290 Å and an angle of 87.7 • between the axis and the centroids of the triazinic ring ( Figure 2C). By contrast, compound 11 exhibited herringbone aggregation with a distance of 3.250 Å and an angle F I G U R E 2 (A) Phosphorescence spectra of compounds 5-9 before (red lines) and after (blue lines) photo-activation under ambient conditions. Inset: Corresponding photographs of compounds 5-9 after photo-activation. [57] (B) Molecular packing modes of compounds 5 and 8 in the side view and top view. [57] Reproduced with permission: Copyright 2018, Wiley-VCH GmbH. [57] (C) Fragment of the crystal packing of 10 (left) and 11 (right). [25] Reproduced with permission: Copyright 2017, American Chemical Society. [25] (D) The carton modes for the packing style of compounds 12-14 in crystal. [37] Reproduced with permission: Copyright 2017, Wiley-VCH GmbH [37] of 50.7 • ( Figure 2C). Therefore, it suggested the strong π-π interactions in H-aggregation contributed to UOP. Similarly, Lu and coworkers reported three pyrimidine-based organic luminogens with ultralong lifetimes of up to 1.37 s and phosphorescence efficiency of 23.6 % due to intense π-π interaction. [58] The overlap and distance between π-π packing molecules lead to the conjugate degree and energy level, thus resulting in different phosphorescence properties. Li and coworkers discovered that compact face-to-face packing of compounds 12-14 is favorable to their UOP properties ( Figure 2D). [37] Molecular energy levels and electrical density contour calculated according to density functional theory revealed compact face-to-face packing can remarkably reduce the excited energy level and facilitate electron communication between dimers. By tuning the substituent groups on the phenyl group, 9,9-dimethylxanthene-derivatives showed increased phosphorescence lifetimes from 52 ms to 601 ms, [59] which can be ascribed to the intense π-π packing. Furthermore, based on seven 10-phenyl-10H-phenothiazine-5,5-dioxide derivatives (15)(16)(17)(18)(19)(20)(21), [9] they demonstrated that the electron-withdrawing substituents (-Br, -Cl, -F) could enhance UOP by decreas-ing the π-electron density of π-π interactions, where the electron-donating substituents (-CH 3 , -OCH 3 ) can weaken UOP through hindering π-π interactions via high π-electron density ( Figure 3A).
To promote Φ P , the introduction of lone-pair electrons (n) in organic molecules is an effective strategy to enhance the ISC process. [55] In 2016, Chi and coworkers proposed that intermolecular electronic coupling units (n and π units) can improve Φ P . [43] The carbonyl groups in 22 and 23 are parallel stacking to the carbazole groups with short distances of 3.373 Å and 3.561 Å (Scheme 2 and Figure 3B). This intermolecular electronic coupling significantly increased intermolecular ISC channels, leading to bright UOP. This concept was also applied to compounds 24 and 25 (Scheme 2 and Figure 3B). Based on the intermolecular electronic coupling of organic Depiction of the electrostatic model of substituent effects on π-π interactions from the Hunter-Sanders model: electron-withdrawing substituents could enhance π-π interactions by decreasing the π-electron density of the substituted π-system and relieving the π-π repulsion, while electrondonating substituents hinder π-π stacking through the opposite mechanism. The dimers with π-π stacking interactions of compounds 16-18 in crystal. Inset: Photographs of compounds 15-20 in crystal taken under 365 nm lamp off. Reproduced with permission: Copyright 2018, Springer Nature. [9] (B) Intermolecular electronic coupling of the n and π units in single crystals of 22, [43] 24, [43] 26 [60] and intramolecular electronic coupling of π units in a single crystal of 27. [61] Inset: Photographs of compounds 26 in crystal taken under 365 nm lamp on and off. Reproduced with permission: Copyright 2016, Wiley-VCH GmbH. [43] Reproduced with permission: Copyright 2018, The Royal Society of Chemistry. [60] (C) Photographs of compounds 27 in crystal excited by 365 nm and 808 nm respectively. [61] Reproduced with permission: Copyright 2019, The Royal Society of Chemistry. [61] (D) The triplet state for carboxyl groups with a hybrid mixture of an electronic configuration the two configurations with different proportions of (n, π*) and (π, π*). [55] Reproduced with permission: Copyright 2016, Elsevier Inc [55] S C H E M E 2 Chemical structures of ultralong room temperature phosphorescence materials based on n-π interactions

S C H E M E 3 Chemical structures of ultralong organic phosphorescence (UOP) materials containing halogen bonding interactions in their crystals
units, Mu et al. designed an organic compound 26, [60] where the plane of the xanthone group was parallel to the plane of the carbazole group in neighboring molecules with short distances of 3.323 Å and 3.370 Å. Later, compound 27 with two-photon absorption was reported due to intramolecular space charge transfer ( Figure 3C). [61] To balance phosphorescence lifetime and efficiency for optimal UOP property, Tang and coworkers reasonably adjusted the ratio of n-units and πunits in organic molecules. [55] The coexistence of n-orbital and π-orbital produce a hybrid excited state with (n, π*) and (π, π*) configurations. The relatively smaller energy gap and larger α n value facilitate ISC to populate the triplet state. The large value of β π in T 1 suggests its high 3 (π, π*) feature and hence its slow k P and long τ P . The appreciable proportions of 3 (n, π*) (α n ) and 3 (π, π*) (β π ) configurations in the S 1 and T n states are desired for obtaining efficient persistent RTP materials ( Figure 3D). Through tailoring the aromatic subunits in arylphenones (28)(29)(30)(31), compound 30 shows a long lifetime of 232 ms and a high Φ P of 34.5%.

Halogen bonding
Heavy-atom effect can enhance SOC and increase the rate of ISC. [8] According to perturbation theory, the perturbation factor δ presents singlet-triplet mixing. A greater spin-orbit matrix value and a smaller energy gap ∆E ST contributes to a larger δ. The spin-orbit matrix elements are highly sensitive to the atomic number of the atoms in the vicinity of the exciton. SOC is proportional to the fourth power of atomic number, Z 4 . Since the internal heavy-atom effect was discovered by McClure in 1949, halogen atoms have been incorporated into organic molecules to enhance Φ P . [62] In 2011, Kim and coworkers reported high Φ P of up to 55% via heavy atom effect from halogen bonding between the aromatic aldehy-des and the bromine atom in the mixed crystal. [63] Based on this concept, Wang and coworkers also prepared mixed crystals of compounds 33 and 32 (1:100, mass ratio), which showed greenish afterglow with a long lifetime of 173 ms and high Φ P of 17%, while individual crystalline 33 and 32 compounds exhibited a deep-blue emission with a lifetime of several nanoseconds (Scheme 3). [64] Because mixed crystals of 33/32 possessed a rigid structure and Br-rich environment for halogen bonding. It is rare to study the influence of various halogen bonding on phosphorescence. Cai et al. prepared isomeric organic phosphors (36)(37)(38). Diverse crystal stacking generated various halogen bonding due to different steric hindrances. Effective π-type halogen bonding with short distances and favorable coupling angles facilitated SOC for a high Φ P of 13% ( Figure 4A). [16] Liu and coworkers successfully realized bright red UOP (40). [65] The C-Br bond directs vertically to the adjacent carbazole plane with a short distance of 3.429 Å, thus generating a high Φ P of 11% ( Figure 4B). Yin et al. proposed a strategy to obtain high Φ P by bridging the carbazole and halogenated phenyl ring with a methylene linker. [66] A Φ P up to 38% was obtained for 52 with a lifetime of 220 ms. Intermolecular π-Br interactions accelerated the ISC process, while tetrahedron-like structures induced by sp 3 methylene linkers restrained the non-radiative decay channel, leading to the high Φ P . Compared with intermolecular hydrogen bonding, the introduction of heavy atoms in a molecule skeleton is much more controllable for the heavy atom effect. Shi et al. proposed an intramolecular space heavy-atom effect to improve phosphorescence efficiency. Compound 39 with two bromine atoms was designed. In a single crystal, there exist strong intramolecular halogen bonding with short distances of 3.13 Å and 3.59 Å ( Figure 4C). A high Φ P can reach up to 38.1%. [44] After the removal of the bromine atoms, the phosphorescence efficiency is only 2.5%, which was ascribed  [69] to the disappearance of the heavy-atom effect. Based on intramolecular halogen bonding, Chi and coworkers synthesized four isomers (47)(48)(49)(50) with Br substituted at different positions, [67] where strong intramolecular halogen bonds (C-Br⋅⋅⋅O=S) were the main driving forces for UOP with high Φ P ( Figure 4E). The ortho-substituted compound 49 exhibited an ultralong lifetime of 180 ms and high Φ P up to 52.1% compared to others. Intramolecular triplet-triplet energy transfer (TTET) can also boost the Φ P . [68] With an additional bromine atom, 42 and 44 showed stronger phosphorescence with higher Φ P of 41.2% (τ P = 0.54 s) and 12.1% (τ P = 0.42 s) respectively (Scheme 3), due to the heavy atom effect caused by bromine atom. Moreover, a facile strategy of heavy atom-participated anion-π + interactions is proposed to construct RTP-active organic salts (45, 46) (Scheme 3). [69] Those compounds with heavy-atom counterions (bromide and iodide ions) exhibit outstanding RTP due to the external heavy atom effect via anion-π + interactions ( Figure 4E). Therefore, intra/intermolecular halogen bonding can promote the ISC, thereby enhancing Φ P .

Hydrogen bonding
Hydrogen bonding can confine the molecular motions in molecular crystals, which provided a rigid environment to

S C H E M E 4
Chemical structures of ultralong organic phosphorescence (UOP) materials containing strong hydrogen bonding interactions in their crystals effectively suppress the non-radiative transition of triplet excitons, thus improving phosphorescence properties. [46] Hydrogen bonds have unique properties such as stability and directionality as well as electrostatic nature, they play a great role in crystal engineering. In 2010, Yuan et al. reported crystallization-induced phosphorescence (CIP) in benzophenone and its derivatives. [17] Initially, these compounds emitted no phosphorescence in solutions or doped in a rigid matrix; while after crystallization, they showed bright phosphorescence. Based on the concept of CIP, they observed UOP in pure organic aromatic acids (53) (Scheme 4). [18] There are numerous C=O⋅⋅⋅H-O (1.623 Å, 1.703 Å), H-O⋅⋅⋅H-C (2.662 Å) interactions as well as partial π-π interactions (3.396 Å) in the acid crystals to form two-dimensional (2D) and three-dimensional (3D) rigid networks ( Figure 5A), which effectively decreased molecular motions and non-radiative dissipations. The lifetime of 53 in the crystal can reach up to 1.24 s under ambient conditions. Besides aromatic acid, Li and coworkers discovered that phenylboronic acid and its derivatives (54-58) demonstrated UOP with an ultralong lifetime of 2.24 s. [46] The multiple hydrogen bonds (BO-H⋅⋅⋅H-O, 1.991 Å, 2.196 Å; C-O⋅⋅⋅H-O, 2.718 Å) restricted intramolecular rotation, thus decreasing non-radiative transition for an ultralong lifetime ( Figure 5B). Fluoro-substituted phenylboronic acid derivatives (59-62) also showed UOP. The lifetime can reach up to 2.50 s due to H-aggregation and rotation confinement via multiple hydrogen bonds. [47] Moreover, non-aromatic organic acid also demonstrated UOP due to multiple hydrogen bonds. Fang et al. reported a non-aromatic organic compound 63 with a long lifetime of 862 ms. [70] The strong intermolecular hydrogen bonds are the main driving force for their phosphorescence emission. Chen and coworkers successfully achieved multicolor circularly polarized organic afterglow (CPOA) through chiral clusterization of 64. [71] Multiple hydrogen bonds in 64 crystals could not only promote the formation of rigid molecular configuration for suppressing non-radiative transitions ( Figure 5C) but also facilitate through-space conjugation (TSC) among different molecules. Non-conjugated clusters via TSC contributed to highly efficient and significantly tuned CPOA emissions from blue to yellowish-green with a dissymmetry factor over 2.3 × 10 −3 and a lifetime of up to 587 ms. With the distinguished color-tunable CPOA, the multicolor displays and visual RTP detection of ultraviolent light wavelength are successfully constructed ( Figure 5D). Other non-aromatic organic compounds (65-71) with strong hydrogen bonding (N-H⋅⋅⋅O=C) showed color-tunable UOP due to the coexistence of diverse clustered chromophores ( Figure 5E). [72,73] Hydrogen bonds between two compounds contributed to superior UOP properties. Interestingly, Bian et al. constructed three supramolecular frameworks (72/53, 72/73, 72/74) through the self-assembly of melamine and aromatic acids. [45] Multiple intermolecular interactions (N-H⋅⋅⋅O=C, N-H⋅⋅⋅N) in cocrystals provided a rigid environment to lock the molecules ( Figure 5F), which not only decreased the nonradiative decay of triplet excitons but also promoted electronic communications. Such supramolecular networks achieved an ultralong lifetime of up to 1.91 s and a high Φ P of 24.3% under ambient conditions, simultaneously.
Hydrogen bonds can be divided into strong hydrogen bonds (such as N-H⋅⋅⋅O, O-H⋅⋅⋅O) and weak hydrogen bonds (such as C-H⋅⋅⋅O, C-H⋅⋅⋅π). Weak hydrogen bonding can also improve UOP properties. Gu et al. reported a series of weak hydrogen-bonded organic molecules (75-77) with color-tunable UOP in single-component molecular crystals Scheme 5. The emission color changed from violet (380 nm) to green (505 nm) under ambient conditions. [74] In the crystal of compound 75, each molecule was fixed by six adjacent molecules through multiple hydrogen bonds (C-H⋅⋅⋅N) with different short distances of 2.634 Å, 2.684 Å, and 2.757 Å to form a rigid 2D network in the same plane ( Figure 6A). The restriction of molecular movement at a single molecular level contributed to an isolated molecule with blue UOP. By tailoring molecular structures, the lifetime and Φ P can reach up to 2.45 s and 31.2% respectively under ambient conditions. Tian et al. reported a series of phenothiazine compounds (80-  [46] (C) Intermolecular interactions of dimer in 64 single crystal. (D) Color-tunable circularly polarized organic afterglow display of COO pattern using 64 to fabricate "C" and 64 for "OO" and the corresponding CPL curves of "C" (black) and "O" (red) in the chiral-featured pattern upon 254 (left) and 365 (right) nm UV light excitation. Reproduced with permission: Copyright 2022, Springer Nature. [71] (E) Intermolecular interactions of 68 in crystals and structure pattern of 73 under 254 nm lamp off and 365 nm lamp off. Reproduced with permission: Copyright 2020, The Royal Society of Chemistry. [72] (F) Molecular packing along the a-axis of 72/53. Inset: Photographs of cocrystal 72/53 taken under 365 nm lamp on and off. Reproduced with permission: Copyright 2018, American Chemical Society [45] 83) with UOP. [49] The lifetime of compound 83 is 876 ms, 19 times longer than that of compound 81. Single crystal analysis demonstrated that compound 83 had more and stronger hydrogen bonds (C-H⋅⋅⋅O) ( Figure 6B), which provided a rigid molecular environment to enable longlived phosphorescence. In 2020, Huang and coworkers reported a twisted donor-acceptor architecture (91) with high Φ P up to 45%. [50] via hydrogen bonding C-H⋅⋅⋅F-B ( Figure 6C).
Besides small organic compounds, hydrogen bonding also plays an important role in polymers with high UOP performance. Tian and coworkers realized decent Φ P and ultralong lifetime in the amorphous state by copolymerization of acrylamide with different phosphors ( Figure 7A). [48] Strong hydrogen-bonded networks among polymeric chains sup-  [48] Reproduced with permission: Copyright 2020, Springer Nature. [78] (B) The synergistic enhancement strategy for ultralong and efficient roomtemperature phosphorescence. Reproduced with permission: Copyright 2020, Wiley-VCH GmbH. [26] (C) Chemical structure of polymer 96 and photographs of 96 in powder under UV light on and off. Reproduced with permission: Copyright 2018, Wiley-VCH GmbH. [79] (D) Chemical structure of chromophore 97 and polymer poly(vinyl alcohol) (PVA). [21] (E) Proposed intermolecular interactions between compound 97 and polymer PVA. Inset: photograph of the polymer film 97/PVA under UV lamp off. [21] Reproduced with permission: Copyright 2018, the American Association for the Advancement of Science [21] ization (93) ( Figure 7A). [78] Their phosphorescence color spanned from blue to yellow with a long lifetime of 1.2 s and a high Φ P of 37.5%. Polyacrylic acid not only enhanced SOC by improving the ISC from S 1 to T 1 but also suppressed the non-radiative decay of triplet exciton via hydrogen bonding networks to restrict molecular motions. Impressively, Liu and coworkers proposed a synergistic enhancement strategy to achieve an ultralong lifetime of up to 2.81 s and a high Φ P of 76% in the polymer ( Figure 7B). [26] The copolymerization between phosphors and acrylamide (94, 95) provided multiple hydrogen bonds to lock the chromophores and carbonyl to promote ISC, prolonging the lifetime from 1.66 ns to 2.46 s and enhancing Φ P from 0 to 57%. After complexing with cucurbit [6,7,8] Figure 7C), [79] which S C H E M E 6 Chemical structures of ultralong organic phosphorescence (UOP) materials containing coordinative bonding in their crystals was attributed to the formation of inter-/intrahydrogen bonding between the sulfonic acid groups. Besides copolymerization with acrylamide or polyacrylic acid, poly(vinyl alcohol) (PVA) is also used as a rigid matrix to suppress the nonradiative transitions of the phosphors. For example, compound 97 containing six benzoic acid arms did not show UOP ( Figure 7D). [21] After embedding it into PVA, plenty of hydrogen bonds between the carboxyl group of compound 104 and host PVA effectively suppress nonradiative relaxation pathways of triplet excitons ( Figure 7E), resulting in an ultralong phosphorescence lifetime of 0.28 s.

Coordinative bonding
Coordinative metal can facilitate the SOC via heavy atom effect, and coordinative bonds can rigidify the molecular conformations to restrict the non-radiative transition of triplet exciton, thus prolonging the phosphorescence lifetime. In 2016, Yan and coworkers first reported a series of MOFs (98-103) with an ultralong lifetime of 1.3 s (Scheme 6 and Figure 8A-C). [32] Terephthalic acid as an aromatic ligand had low Φ P (0.5%) and a short lifetime (2.86 ms). After coordination with Zn 2+ , MOFs 98 exhibited a higher Φ P of 3.4% and a longer lifetime of 475 ms. Apart from intermolecular interactions, the packing of organic ligands was similar to the pristine terephthalic acid. Therefore, the coordination bonds played a crucial role in UOP. Similarly, compared with the pristine organic ligands, the lifetimes of MOFs 98 and 100 were increased significantly. Later, Yan and coworkers synthesized one-dimensional metal-organic halide microcrystals 104 and 105 with highly tunable multicolor afterglow. [80] Both 104 and 105 demonstrated mononuclear and binuclear metal-organic halides in a monoclinic crystal system ( Figure 8D). Taken 104 as an example, mononuclear and binuclear metal-organic halides showed green and blue ultralong RTP, respectively. They found that the shorter π-π interaction distances and the parallel arrangement of the organic chromophores endowed the mononuclear complexes with red-shifted phosphorescence emission compared with that of the binuclear complexes. Interestingly, after the Zn 2+ ion was replaced by the Cd 2+ ion, 2D organic-metal halide perovskite materials of 106 and 107 showed blue phosphorescence emission with peaks at 444 nm at room temperature. [81] With density functional theory calculation, they found that both strong 2D confinement effect and intermolecular π-π stacking of inorganic-organic quantum well structure made a contribution to phosphorescence generation by stabilizing triplet excitons. More recently, Liu et al developed a series of ligand-functioned MOF (108-110) with highly efficient blue UOP under ambient conditions ( Figure 8E-G). [82] They found that the guests with static effects that liked brakes rendered the luminescent ligands with efficient UOP by coordination and host-guest interactions. MOF 110 with diethylamine cation showed highly efficient blue phosphorescence up to 80.6% and a lifetime of 169.7 ms under ambient conditions.  [32] (D) Afterglow images and single crystal data of 104 with mononuclear (left) and binuclear (right) metal-organic halides in crystal. Reproduced with permission: Copyright 2021, Wiley-VCH GmbH. [80] (E) Crystal data of 109 and 110, and photographs of 109 and 110 taken under a 302-nm lamp on and off. [82] (F) Lifetime profiles of the phosphorescence detected at 411 nm for 109 and 110. [82] (G) Absolute phosphorescence quantum efficiencies of 109 and 110. [82] Reproduced with permission: Copyright 2022, Wiley-VCH GmbH. [82] (H) (Ph 4 P) 2 CdX 4 (top) and (Ph 4 P) 2 Cd 2 X 6 (bottom) with C-H⋅⋅⋅π interactions in 111 and 112 crystals. [83] (I) The emission spectra of 112 in the solid state excited by 300 nm at temperatures from 100 to 400 K. [83] Reproduced with permission: Copyright 2020, Springer Nature. [83] (J) Crystal data of 115. [84] (K) Images of 115, 116, and 117 in crystals under a 320-nm lamp on and off. [84] Reproduced with permission: Copyright 2020, Wiley-VCH GmbH [84] Recently, Yan and coworkers creatively designed thermalquenching resistant phosphorescent materials via an effective intermediate energy buffer and energy transfer route. [83] Metal halide organic-inorganic hybrids 111 and 112 had zero-dimensional (0D) single-nuclear (CdX 4 2− ) and dinuclear (Cd 2 X 6 2− ) clusters in crystal ( Figure 8H). Impressively, metal halide organic-inorganic hybrids 112 exhibited luminescent stability across a wide temperature range from 100 to 320 K (ΔT = 220 • C), which showed high phosphorescence efficiency of 62.79% and a lifetime of 37.85 ms at room temperature ( Figure 8I). Ma and coworkers reported a series of 0D tetraphenylphosphonium (TPP + ) metal halide hybrids (113-117) with other metals (Sb, Mn, and Zn) ( Figure 8J). [84] 113 shows red emission peaked at 645 nm with a photoluminescence quantum yield (PLQY) of 85% and a lifetime of 4.6 ms, while 114 has green emission peaked at 517 nm with a PLQY of 48% and a lifetime of 1.97 ms, respectively. For 115, the afterglow is up to 6 s, which was ascribed to stronger π-π stacking and intermolecular electronic coupling between TPP + cations in crystal. The afterglow was weakened with the change from Cl (115) to Br (117) due to the heavy atom effect ( Figure 8K).
Besides the above metal-organic halide hybrids, organicinorganic hybrid perovskites also demonstrated ultralong RTP. Hu et al. first reported a series of mixed-cation perovskites with RTP (118-123, Figure 9A,B). [85] When part of the cations in perovskite 119 was replaced with 118, 120, 121, 122, and 123 cations, the mixed-cation perovskites displayed distinctive phosphorescence colors. impressively, the chlorine perovskite with 121 cation dopant showed a long lifetime of 71 ms. The afterglow can be observed by the naked eye. Recently, Lin and coworkers designed 0D per-  [87] ovskites (124 and 125) with blue RTP based on InCl 3 and aniline hydrochloride ( Figure 9D,F). [86] 125 demonstrated a high RTP quantum yield of 42.8% and an ultralong RTP lifetime of up to 1.2 s ( Figure 9E). Furthermore, white RTP can be realized when Sb 3+ was doped into perovskites 125 ( Figure 9F). Du and coworkers designed and synthesized two lead-based organic-inorganic metal halides with RTP (126 and 128). [87] The crystal structures of 126 and 128 have been shown in Figure 9G,H. All lead-based organicinorganic metal halides demonstrated bright luminescence under the UV lamp and weak afterglow when the UV lamp was switched off ( Figure 9I).

Ionic bonding
An ionic bond is a kind of electrostatic interaction between an anion and a cation, widely used in hydrogels and supramolecular polymers. Compared with the hydrogen bond, the ionic bond possesses the unique characteristics of strong interactions, directionless, and non-saturation. Ionic bonding is stronger than hydrogen bonding, [42] which can also provide a rigid environment to minimize the non-radiative decay of triplet excitons. Huang and coworkers reported the first ionic crystals with UOP via ionic bonding ( Figure 10A). [51] With the variation of cations from NH 4 + to Na + , K + , the emission color of UOP was tuned from yellow-green to sky blue ( Figure 10A). Taking compound 130 as an example, ammonium ions induced face-to-face stacking of benzene chromophores with a distance of 3.510 Å. Moreover, ionic bonding between ammonium cations and terephthalate anions locked the terephthalate anion conformation and restricted the molecular motions and vibrations, thus suppressing the non-radiative decay of triplet excitons. K + and Na + ions had a similar function for decreasing non-radiative transition, thus contributing to UOP. Three years later, a high Φ P of 96.5% was achieved by confining isolated chromophores in ionic crystals. [88] Multiple ionic bonds between the cations and carboxylic acid groups led to a segregated molecular arrangement with negligible inter-chromophore interactions ( Figure 10B). Phosphorescence emission colors from blue to deep blue with a maximum Φ P of 96.5% can be achieved by varying the charged chromophores and their counterions (133-137) ( Figure 10C, D). Chen et al. reported anion-regulated persistent phosphorescence based on tetraphenylphosphonium cations (138-140). [52] Compound 140 had high emission efficiency of 56% and size-dependent afterglow. The theoretical calculation revealed significant charge transfer between separated locations of the highest occupied molecular orbitals and least unoccupied molecular orbitals in ionic compounds enhancing the ISC process and phosphorescence proportion. Strong ionic interactions between ClO 4 − and TPP cations also facil-  [88] itated the generation of the afterglow ( Figure 11A). She et al. proposed an effective strategy to achieve tunable UOP by manipulating the external heavy-atom effect. [89] They designed and synthesized a series of triphenylphosphonium derivatives (145-147) with different halide anions (Cl − , Br − and I − ). The emission color of compound 145 was tuned from blue fluorescence to UOP by changing the counter anions to Br − or I − . Theoretical calculations demonstrated that intense ionic halogen bonding plays a dominant role in generating UOP. Tang and coworkers also reported a series of phosphonium bromide salts (141-144) with ultralong phosphorescence in a crystal at room temperature. [53] These phosphors have long-lived lifetimes of more than 100 ms ( Figure 11B). Crystal data revealed that C-H⋅⋅⋅π and C-H⋅⋅⋅Br interactions in their crystals suppressed molecular motions, thus enhancing phosphorescence efficiency ( Figure 11C). Impressively, after a mixture of compound 143 and N, Ndimethylaniline, afterglow emission of over 7 hours was observed after the stoppage of excitation. Moreover, Tang and coworkers discovered crown ether with phosphorescent properties. The lifetime of crown ether can be prolonged via chain length adjustment and complexation with potassium (148-151) (Scheme 7). [54] For example, the phosphorescence lifetime of compound 148 increased by ten times compared with the crown ether ( Figure 11D). After complexing with potassium ion, multiple C-H⋅⋅⋅π interactions and sandwiched conformation fixed the core chromophore, which create a more rigid microenvironment to suppress molecular motions. Huang and coworkers presented a concise chemical strategy to achieve UOP in polymers by ionic bonding cross-linking (152-163) (Scheme 7 and Figure 11E). [90] The lifetime of ionic polymer can reach up to 2.1 s. When ionic bonds were destroyed by moisture, the lifetime of the ionic polymer decreased sharply. The optimized molecular structure of polymer 153 also revealed the existence of ionic bonding cross-linking between the chromophores, which played a critical role in suppressing non-radiative transitions for UOP. The emission of ionic polymers can be tuned by changing the excitation wavelength ( Figure 11F). In addition, ionic polymers can remain phosphorescent at high temperatures due to strong ionic bonding. An and coworkers adopted a concise chemical ionization strategy to endow conventional poly(4-vinylpyridine) derivatives with UOP. [91] After the ionization of 1,4-butanesultone, polymers 164-166 showed UOP with a long lifetime of 578.36 ms, 525 times longer than that of the original polymer. Remarkably, multicolor UOP emission ranging from blue to red was achieved by varying the excitation wavelength.

SUMMARY AND OUTLOOK
The development of UOP materials has made great progress in recent years, especially in small organic compounds. The lifetime can reach up to 4.17 s, organic afterglow can last for 7 h, and phosphorescence efficiency can reach up to 96.5%. In this review, we systematically discussed the intrinsic relationships between intermolecular interactions and UOP properties. It was divided into six parts, namely π-π interactions, n-π interactions, halogen bonding, hydrogen bonding, coordinative bonding, and ionic bonding. The π-π interactions can not only stabilize triplet excitons but also tune the electronic coupling degree, thus contributing to long lifetime and dynamic phosphorescence properties. n-π interactions  [90] and halogen bonding can promote SOC, thus facilitating ISC, which effectively enhances phosphorescence efficiency. Strong hydrogen bonding effectively decreased the nonradiative transitions caused by the molecular motions and external environment, prolonging UOP lifetime in organic materials. Coordinative bonding and ionic bonding can form a more rigid environment to improve UOP performance. We anticipate this review will provide speculative knowledge for designing UOP materials with excellent properties. To further improve the UOP performance by manipulating intermolecular interactions, more effort is necessary to be devoted to the following aspects in the future. Firstly, new materials with multiple-type intermolecular interactions in crystals need to be developed for understanding the relationship between molecular structures and UOP performance. For instance, apart from π-π interactions of the chromophores, hydrogen bonding, coordinative bonding, or ionic bonding can be rationally introduced into molecular architectures for the self-assembly of chromophores, which is beneficial to obtain predictable UOP performance. Notably, amorphous materials or polymers with UOP features attract great attention owing to their flexibility and processability in practical applications. However, it is still a formidable challenge to exactly clarify the interactions between chromophores. So advanced techniques and theoretical simulation, even machine learning are urgently needed to reveal the effect of intermolecular interactions on UOP performance at the electronic, molecular and aggregate levels. Moreover, the realization of dynamic UOP behaviors will greatly promote the development of RTP materials for practical applica-S C H E M E 7 Chemical structures of ultralong organic phosphorescence (UOP) materials containing ionic bonds in their crystals tions by regulating intermolecular interactions with external stimuli, such as light, heat, force, etc. In a word, it is a promise but a challenge to develop high-performance UOP by manipulating intermolecular interactions.

C O N F L I C T O F I N T E R E S T
The authors declare no conflict of interest.